Acceleration dependent phase control seismic exploration



2 Sheets-Sheet 1 H. B. MORRIS Jan. 13, 1970 ACCELERATION DEPENDENT PHASECONTROL SEISMIC EXPLORATION Filed Dec. 31, 1968 FIG.I

INVENTOR;

HAROLD B. MORRIS $4M, Y/M

;! MMA,

ATTORNEY H. B. MORRIS Jan. 13, 1970 Filed D ec.

2 Sheets-Sheet 2 M2 N2 m9 n l lllllllll \5 mm LI vv 8 I l r an l t E Q 9mm m l mu 8% 2 w m m I O V 0? mm |\|||\/I Q G l 0 H r mm w r mm cm 6 a?L ofl mm vn wm H 09 3 w 55:26 I. E V E k R wk 5960mm NEE United StatesPatent 3,489,997 ACCELERATION DEPENDENT PHASE CONTROL SEISMICEXPLORATION Harold B. Morris, Houston, Tex., assignor to ElectronicSystems, Inc., Houston, Tex., a corporation of Texas Filed Dec. 31,1968, Ser. No. 799,139 Int. Cl. G01v 1/04 US. Cl. 340-15.5 6 ClaimsABSTRACT OF THE DISCLOSURE Seismic waves are detected and recorded bygenerating acceleration signals through prestressed piezoelectriccrystals of wide band frequency capability and passing the wide bandsignal to a multi-stage amplifier having wide band input andprogressively narrowed frequency band through successive stages of theamplifier while retaining signals of frequency in the range of about 100cycles per second and above.

This invention relates to seismic exploration and more particularly to ahigh frequency system having identical wide band inputs on each of aplurality of channels with identical progressive low frequency filteringalong each channel, with wide band signal generation by a high impedancetransducer.

In seismic exploration it has been the practice to concentrate interestand attention to the low frequency portion of the seismic spectrum. Ingeneral, the frequencies of interest have been below 100 cycles persecond with various provisions being made for filtering to select narrowband information for recording and display. Accordingly, time scales onseismic records in seismic record sections have been of the order ofinches per second or less with timing markers or timing lines on suchrecords being spaced at 0.01 second apart with resolution generally ofthe order of about 0.001 second. This approach has carried over intoautomated data processing operations. More particularly, in digitalseismic systems the seismic signals are digitized at 0.001, 0.002, 0.003or 0.008 second sample intervals.

It has further been considered that the earth is not responsive tohigher frequencies and thus, except for operations in areas where onlyshallow beds having sharp boundaries are of interest, the high frequencyportion of the spectrum has not been used.

Where attempts have been made to use the high frequency portion of thespectrum, ultimate recordings or displays have been found to becharacterized by erratic appearance of energy which might berepresentative of reflections from a given boundary. In general therehas been inability consistently to secure time coincidence betweenportions of the signals of given character as to identify a reflectingbed.

The present invention is directed to a system which has been found tohave overcome the problems present in prior operations. It is believedthat the results achieved by the present invention in a combination of aplurality of elements and steps which in conjunction one with the othercooperate to permit the reliable recording of energy reflected from agiven subsurface horizon in such a manner as to permit analysis of theresultant record confidently to postulate the character of thereflecting horizon from signals of the frequencies predominately above100 cycles per second.

More particularly, in accordance with the present invention there isprovided a plurality of seismic signal channels each having a firststage including a high impedance accelerometer with provision formaintaining wide band capability to apply to each channel a signal whichcontains all acceleration energy sensed by the detector from near zerofrequency to a frequency of the order of 1000 cycles per second or more.Each channel is provided with a plurality of identical amplifyingstages, each stage being characterized by the progressive raising of thelower end of the pass band to limit the output signal from each channelto frequencies generally above cycles per second. Each channel has aseparate programmed gain variation in each stage and provision forstoring or recording the signals as to permit resolution of the highfrequency components on the record or storage time scale.

In a more specific aspect, a multistage piezoelectric seismometer with 2contained amplifier stage is provided for producing an output signal ofwide band character. An RC filter is provided at the output of the firststage and of each succeeding stage progressively to raise the cutoflpoint of the lower end of the frequency spectrum.

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference may now be had to thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIGURE 1 is a sectional view of the seismometer employed in accordancewith the present invention,

FIGURE 2 is a sectional view taken along line 22 of FIGURE 1, and

FIGURE 3 is a circuit diagram showing one channel in detail with theremaining channels being indicated diagramatically.

Referring now to FIGURES 1 and 2, the seismometer 10 which forms theinput to each channel of the present system provides a wide bandacceleration dependent seismic signal, with the case 10a, top closure11, bottom closure 12 and a spike 13 threadedly engaged by the bottomclosure 12. A spike 13 is to be used to provide the engagement of thedevice in the ground, and any suitable type of engaging device may beused Without departing from the present invention. The accelerometerwill be mounted in accordance with established knowledge for theparticular application for which it is to be used. The top 11 isprovided with a suitable plug 14 which extends therethrough to conductthe three leads 15, 16 and 17 from the interior to the seismometer 10.

It is generally preferred that the case of seismometer 10 be made from amaterial, such as a rigid cylinder of a material such as a glass-epoxylined with copper, which will provide both thermal and electricalinsulation, and an electrostatic shield for the device. The case ispreferably rigid with very low thermal coeflicient of expansion toshield the same from inputs due to pressure and temperature inducedchanges in the case. It is desired that only acceleration produces aninput. When case 10 is made of such material. it will prevent distortionof the output signal from the device. However, other suitable materialsmay be used.

Within the case of accelerometer 10 are disc support elements 18 bondedto the interior. Support elements 18 are spaced around the middleinterior of the case and are centrally located with respect to thelongitudinal axis of the case. It is preferred that only three or fourof such support elements 18 be used for each supported disc for thereasons hereinafter more fully explained. Such support elements 18 maybe made from a relatively hard plastic material, such as vinyl, nylon,Teflon, or other material, which will not readily recover when deformed.The elements 18 are suitably secured to the interior of the casing 10with dowel pins (not shown) or by a suitable cement which will retainits bond when the accelerometer is subject to vibrations of the naturewhich are to be measured. In accordance with one method of construction,small mounds of silicone rubber are applied to about four places aroundthe edge of the discs. After the silicone rubber cures or sets up, thediscs are positioned in the case. More silicone rubber is applied to thejunction of cured silicone rubber and case. The silicone rubber wedgesthe discs to the case, thus permitting adjustment and retaining exactpositioning of the discs. Further, the little bumpers of silicone rubberprotect the discs from side shocks.

Four crystal discs 19 and 19a, 20 and 20a are provided. Two crystals 19and 20 are supported with a small metal disc of shim stock 22, e.g.,beryllium copper, therebetween at a central position and secured by abonding agent. Preferably a conductor such as a silver epoxy is used atthis spot only, but surrounded by a second non-conducting epoxy. Thebond provides an electrical connection between adjacent faces of suchcrystals 19 and 20. With the disc 22 positioned at the center of eithercrystal 19 or 20, a nonconducting bonding agent is ap- .plied to theremainder of the Surface. The two crystals are brought together withtheir outer peripheries held in contact while a ring of thenonconducting cement 23 is forced out from between the crystals to coverthe outer periphery. Excess cement 23 is then removed before allowing itto cure.

The resultant bowing of the crystals 19 and 20 is exaggerated in FIGURE1 for the sake of clarity of the drawing. A slight bending of thecrystals will result from this assembly procedure, and such slightbowing has had the effect of producing a high mechanical advantage. Itprestresses the cement 23 which bonds the outer edges of the crystalsinto a unitary assembly. This distortion of the crystals changes duringdiaphragm action induced by motion of the accelerometer. The changes inbending are 180 out of phase between the crystals 19 and 20. Very lowlevel compression and tension results from longitudinal-type wave actionbefore the transverse surface waves from the periphery of the crystalsbecomes effective. The mechanical advantage of the initial longitudinalwave action can be about one hundred times greater than the subsequentsurface wave action. Cement 23 used in the bonding of the edges andfaces of the crystals 19 and 20 is preferably an insulating epoxy-typecement which hardens into a relatively strong bonding agent to hold theedges of the crystals secure with respect to each other. If desired,more of the disc assemblies may be used and suitably connected in seriescircuit as shown by conductor 19b. This may enhance the sensitivityleading to the assembly formed by discs 19a and 20a.

The epoxy cement 23 will be in tension vertically. Therefore, the epoxyand discs are under mechanical stress which acts as a bias to increasethe sensitivity.

The crystals 19, 20, 19aand 20a are of the type which will provide anelectric signal when they are subject to a strain. Such crystals aregenerally classified as piezoelectric crystals. These crystals should benonhygroscopic. Typical examples of crystals which have been found to besatisfactory for use in the device of the present invention and theirCurie points are: barium titanate, Curie point 120 C.; leadtitanate-lead zirconate, Curie point 300 to 365 C.; and sodium potassiumniobate, Curie point 300 C.

A typical example of size of crystal discs of the lead zirconate-leadtitanate which may be used in the present invention includes leadzirconate-lead titanate crystal discs having an outer diameter one andthree-fourths inches and a thickness of one-hundredth of an inch. Bothfaces of each crystal disc are silvered and the discs are assembled asillustrated in FIGURE 1 whereby the negative side of the discs areadjacent in the assembly for series connection or a negative andpositive adjacent for parallel connection. A suitable wire is alsosecured to each of the outer faces of the discs for series connection,or one wire is secured to the outer faces and a narrow strip of .001"thick shim stock (beryllium copper) contacts the inner faces of thediscs and a wire is soldered to this strip of shim stock for parallelconnection, to provide the leads 24 and 25 necessary to conduct theoutput of the discs to an amplifier 30, hereinafter more fullydescribed, when they are subjected to strain. It is preferred that theleads from the crystal discs be multiple strand wires for greaterflexibility and that the bare ends of the wires which are joined to thediscs be soldered to the silvered faces or shim stock near the peripheryof the discs.

It is noted that a copper cylinder 10b lines the inner Walls of thehousing 10a and a copper disc 11a provides shielding at the upper end ofcylinder 10a. A copper washer 12a shields the lower end of cylinder 10a.The shield elements are electrically common, being connected to theground terminal of amplifier 30 by way of conductor 26. Alternatively,the shield elements may be tied to the cable'shield so that theconnection to the amplifier ground terminal would be made externally.

A rubber diaphragm 27, preferably a silicone rubber diaphragm, issecured at the rim thereof in a suitable cement in which disc 12a isembedded. The interior of housing 10 is filled with silicone compound.The spike 13 is provided with a passage to permit expansion andcontraction of the filler material. The material employed preferably isof the type manufactured and sold by Dow Corning Corporation identifiedas Dow Corning 111 Compound, The silicon compound will protect the unitagainst shock, act as a hydraulic restrain and provide a weight or massagainst which the crystals 19, 20, 19a and 2001 work. For a specialpurpose, silicone oil (less damping), liquid mercury (high sensitivity),etc. could be used as the filler material.

Amplifier 30 is contained within the accelerometer. It is shown in blockform in FIGURE 1, It is shown in detail in FIGURE 3.

Referring now to FIGURE 3, the detector discs 19, 19a, 20, and 20a areshown inside the electrostatic shield and are connected at one terminalto the ground point 26. The upper terminal of the series of crystals isconnected by way of resistor 31 to the base of a transistor 32 which isthe first stage of amplifier 30 which is a series amplifier includingtransistors 33 and 34. The emitter of transistor 32 is connected to thebase of transistor 33 whose emitter is connected to the base oftransistor 34 whose emitter is connected to ground. The collectors oftransistors 32-34 are connected to the positive terminal of battery 35through collector resistor 35a. The battery 35 energizes only theamplifier 30 within the shield. This is the first stage of a three-stageamplifier channel. The base of transistor 32 is connected by way ofresistor 36 to ground and by way of resistor 37, diode 38, resistor 39,and capacitor 40 to the base of transistor 41 which is in the secondamplifier stage of the system.

Resistors 36 and 37 are of very high impedance, of the order of 1000megohms, in order to assure low current noise and good low frequencyresponse of the system.

The juncture between diode 38 and resistor 39 is connected by way ofcondensor 42 to ground and, by Way of resistors 43 and 44 and manualswitch 46, to a relay switch 45. The relay switch is connected toground. The battery 35 is connected at its positive terminal throughresistor 35a to the juncture between resistor 39 and condensor 40, andat its negative terminal to ground.

The base of transistor 41 is connected by way of diode 50 to ground, andby Way of resistor 51 and diode 52 and resistor 54 to the collector oftransistor '41. The collector of transistor 41 is connected to ground byway of resistor 54 and a filter condensor 53, and by way of condensor55, to the base of transistor 56 which provides a third stage ofamplification of the system.

The juncture between resistors 54 and condensor 55 is connected by wayof resistor 60 to the positive terminal of supply battery 61, thenegative terminal of which is connected to ground. The juncture betweendiode 52 and resistor 54 is connected by way of resistor 62 and 63 andmanual switch 46 to relay switch 45.

The base of transistor 56 is connected by way of diode 70 to ground, andby way of resistor 71 and diode 72 and resistor 76 to the collector oftransistor 56. The collector of transistor 56 is connected by way ofresistor 76 and capacitor 75 to ground and, by way of capacitor 77 tothe input of terminal 78, the first channel input of a recorder 79.

The juncture between resistor 76 and capacitor 77 is connected by way ofresistor 84 to the positive terminal of the battery 85, the negativeterminal of which is connected to ground. The juncture between diode 72and resistor 76 is connected -by way of resistors 86 and 87 and manualswitch 46 to relay switch 45. Thus, there is shown in detail a completeamplifier channel wherein each amplifier is provided with its ownseparate power supply wherein each amplifier stage has an output whichis characterized by a capacitive coupling such as the capacitors 40, 55and 77. The capacitive couplings progressively raise the low frequencycutott' point of the channel. The detector system and the first stageamplification provide wide band signal capability.

Channels 78a-78n of recorder 79 are energized by signals from signalchannels to the channel shown in detail in FIGURE 3, the additionalsignal channels being collectively indicated by the reference character100. In accordance with the invention, all of the channels leading tothe recorder have identical circuits.

It will be noted that the recorder further is provided with power from asource 101. Power is applied to the recorder by way of lines 102. Powerapplied to the recorder is controlled by relays actuated in response tothe pushbutton switch 103. The switch 103 actuates a first relay 104which as a part of the recorder serves to start and stop the paper drivein the recorder 79 in accordance with preset limits, by way of channel105. Preferably, a paper drive of about 80 inches per second will beemployed.

A second delay relay 106 serves to control the operation of a blaster107 to detonate a cap 108 a predetermined time interval after the paperdrive on the recorder 79 has been set into motion.

Other poles of this double throw delay relay 106 represent switch 45which opens just before the contacts close to furnish firing current tocap 108. With the switch 45 closed, the current flow through resistors43 and 44 lowers the voltage effective on the amplifier 32-34 so thatthe high energy first break signals detected by the detector will notexceed the capabilities of the amplifier channel. After the switch 45 isopened, by delay relay 106, the voltage on amplifier 32-34 graduallyincreases to raise the gain in each stage and thereby compensate for thedecrease in level of the seismic energy following the detonation of thecap 108.

In accordance with the present invention, a separate gain controlfunction is applied to each of the stages, providing identical operationfor all of the channels.

By providing wide band input it has been found possible to maintain theproper positioning along the time scale of the polarity reversals ofhigh frequency. The high frequency signals can then be displayed showingcoherence of the signal energy from a given reflector. This is believedto be due in the present invention by providing extremely wide bandcapabilities so that the distortion inherent at the lower end of theband is displaced so far from the frequencies ultimately used that thephase distortion, from one channel to another, is the same at highfrequencies. By careful control of the progressive modification of thelower end of the band it is possible to maintain the same phaserelations among channels for all high frequency components.

While the system has been shown in FIGURE 3 as involving a fieldrecording operation which ultimately produces a paper record ofconventional type except for the high speed character thereof and thehigh frequency content of the signals, it will be noted that the systemmay be employed when a digitizer 121 which feeds a tape recorder 122 forthe storage of the wide band signal which may later be progressivelymodified as to frequency content as well as signal level in accordancewith the provisions of the present invention to provide high frequencyseismic record. In the present case, the signal is derived from the ttput of amplifier 30 by way of capacitor 120 connected to the junctionbetween resistor 39 and capacitor 40.

In one embodiment of the invention, the following parameters were foundto be satisfactory.

Resistor 31 15 megohms.

Resistors 36, 37 1000 megohms.

Transistors 32-34, 41, 56 -1 2N3391A.

Resistors 39, 54, 76 2200 ohms.

Capacitor 42 0.2-200 microfarads.

Resistors 35a, 60, 84 560 ohms.

Batteries 35, 61, 12 volts.

Capacitors 40, 55 0.05-0.5 microfarads.

Capacitor 77 0.22-2.2 microfarads.

Resistors 51, 71 180,000 ohms.

Capacitors 53, 75 25 to 250 microfarads.

Resistors 43, 62, 86 2200 ohms.

Potentiometer 44, 63, 87 2000 ohms.

Recorder 79 Honeywell Model 2106 or 1508 Viscorder 0- 600 cycles persecond with electromagnetically damped galvanometers).

A master control for the maximum gain is also superimposed on the systemabove described. The output signal applied to the recorder 79 actuates atransistor 150 by way of channel 151. The nonlinear output of transistor150 is partially checked by the threshold in diode 152 so that a minimumoutput level may be applied to condensor 42 when the master gain controlswitch 153 is closed. Thus there are two gain control functionseffective. One limits the maximum amplitude and is a system control,looping the entire amplifier. The other provides gain control for eachstage independent of the other stages.

What is claimed is:

1. The high frequency seismic detecting-recording systern whichcomprises:

(a) a prestressed assembly of piezoelectric crystals mounted in ahousing coupled to the earth for generating a wide band signalrepresentative of the acceleration of the earth to which it is coupled,

(b) a wide band high gain amplifier for elevating the level of thesignal generated by said detector,

(c) a plurality of stages of amplification connected in series andleading from the output of said wide band amplifier and characterized byinterstage filters which progressively narrow band of frequency derivedfrom the output of said amplifier to a frequency range of the order ofcycles per second.

2. The invention set forth in claim 1 wherein a separate battery supplyis provided for each stage .in the amplifying channel and in which aseparate gain control function is coupled to the output of each stagethereof.

3. The combination set forth in claim 1 wherein said wide band amplifieris located within said housing.

4. The method of seismic exploration which comprises:

(a) converting the acceleration component of seismic waves to wide bandelectrical signals,

(b) applying said signals to a multi-stage high gain amplifier, and

(c) progressively modifying the frequency band by like filters in eachstage of said amplifier while passing 7 8 signals predominantly at afrequency of about 100 References Cited cycles per second and above.UNITED STATES PATENTS 5. The method according to claim 4 wherein a timevarying signal dependent gain control voltage is applied 12/1952 Lee et34O155 to z p l 4 h h 1 f 5 RODNEY D. BENNETT, 111., Primary Examiner eme o o c aim W erein t e ow requency cutoff of the frequency band isprogressively raised by KAUFMAN Assistant Exammar substantially likeamonnts through filtering in each stage US, Cl. X,R. of said amplifier.34017

